section 26 2
Gene Regulation
in
Prokaryotes
597
F I G U R E 2 6 - 4
Terminal region of the trp leader mRNA (right end of L in Figure 26-3).
The base sequence given is extended past the termination site at position
140 to show the long stretch of U’s. The bases (colored lines) form an
inverted repeat sequence that could lead to the stem-and-loop configuration
shown (segment 3^f in Figure 26-5).
sequence quite sensitive to the concentration of charged
tRNATrp. If tryptophan is limiting, there will be insuf-
ficient charged tRNATrp and translation will pause at the
tryptophan codons. Thus, regulation depends on two char-
acteristics of gene regulation in bacteria.
1. Transcription and translation are coupled.
2. Base-pair formation in mRNA is eliminated in any
segment of the mRNA that is in contact with the
ribosome.
Figure 26-5 shows that the end of the trp leader peptide is in
segment
1
and a ribosom is in contact with about
1 0
bases
in the mRNA past the codons being translated. When the
final codons are being translated, segments
1
and
2
are not
paired. In a coupled transcription-translation system, the
leading ribosome is not far behind the RNA polymerase
molecule. Thus, if the ribosome is in contact with segment
2, when synthesis of segment 4 is being completed, then
segments 3 and 4 are free to form the duplex region 3-4
without segment 2 competing for segment 3. The presence
of the 3-4 stem-and-loop configuration allows termination
to occur when the terminating sequence of seven U’s is
reached.
If exogenous tryptophan is not present or is present in
very small amounts, the concentration of charged tRNATrp
will be limiting, and occasionally a translating ribosome
will be stalled for an instant at the tryptophan codons.
These codons are located 16 bases before the beginning of
segment 2. Thus, segment 2 will be free before segment 4
has been translated and the 2-3 duplex will form. In the
absence of the 3-4 stem and loop, termination will not
occur and the complete mRNA molecule will be made,
including the coding sequences for the
trp
genes. Thus,
if tryptophan is present in excess, termination occurs and
little enzyme is synthesized; if tryptophan is absent, there
is no termination and the enzymes are made. At interme-
diate concentrations, the frequency of ribosome pausing
will be such as to maintain the optimal concentrations of
enzymes. This tryptophan regulatory mechanism is called
attenuation
and has been observed for several amino acid
biosynthetic operons, e.g., histidine and phenylalanine.
R ib o s o m e
(a)
Free mRNA. Base pairs
between 1
and 2 and
between 3 and 4.
(b)
Low concentration of
tryptophan. Ribosome
stalled in region 1
permits formation of 2 -3
before transcription of region
4 is completed.
(c)
High .concentration of
tryptophan. Ribosome
reaches region 2 before
region 4 is completed, and
permits formation of 3 -4 .
F I G U R E 2 6 -5
Model for the mechanism of attenuation in the
E. coli
trp operon.
